A conventional plane-surface heater consists of a resistive element that is encapsulated in electrical insulation. The resistive element is either fine wire, such as nichrome or tungsten, or it is a conductive, thin-film metallic or graphite deposition. For high power-density applications, the insulation is usually a ceramic such as mica or alumina. With application of electrical power, current flows in the plane of the resistive element. The resistive element temperature Te rises and heat (q) is conducted across the insulation. The temperature of the resistive element is related to the heat transfer rate as:Te−Ts≈(q/As)((t/k)+R″)where Ts and As are the heater insulation surface temperature and surface area per side, respectively; k and t are the insulation thermal conductivity and thickness, respectively; and R″ is the resistive element-to-insulation contact thermal resistivity. The total thermal resistivity, (t/k)+R″ is a performance limiter since it is usually relatively large.
An example is a mica insulated flat heater (k=0.71 W/mK, t=0.3 mm). At 500 W/cm2 per side power density (q/As), and assuming R″=0, the element temperature rise across the mica insulation would be Te−Ts≈2100° C., which is not feasible. If the resistive element is nichrome, the element would melt. Consequently, the manufacturer limits power density of the mica-insulated plane-surface heater to 17.1 W/cm2 when Ts=150° C., and the maximum permissible power density decreases to zero when Ts=600° C. Similarly, another exemplary plane-surface heating element that includes pyrolytic graphite (PG) encapsulated in pyrolytic boron nitride (PBN) insulation is limited to power densities of less than 50 W/cm2.